A substantial amount of the materials used for carpet underlay are formed from, what is termed, rebonded urethane foam. Rebonded urethane foam is the product obtained when small, shredded particles, e.g. -- one-half inch cube in size, of a flexible urethane foam are coated with a thin layer of a prepolymer adhesive and compressed until the adhesive cures sufficiently to maintain the particles of urethane foam in the compressed state, i.e. -- the product has dimensional integrity or stability.
In manufacturing rebonded urethane foam, typically, the shredded, small particle urethane foam is placed in a suitable mixing container, such as a ribbon blender, where the foam particles are subjected to vigorous mixing. As the shredded foam particles are being agitated, the prepolymer adhesive is sprayed into the mixing chamber where it coats the particles of scrap, flexible urethane foam. Water may also be added to the mixing chamber before the prepolymer is added, at about the same time the prepolymer is added, or after addition of the prepolymer. The water may or may not contain a catalyst to promote the curing of the prepolymer adhesive. After the shredded foam and prepolymer mixture are thoroughly blended, the mixture is transferred from the mixer to a mold and compressed (if a batch process is being used) or to continuosly moving compression conveyors (when the process is "continuous"). In either case, the mixture is held in the compressed state until the shredded foam/prepolymer block achieves dimensional stability. It is this compressed block of shredded foam/reacted prepolymer which is identified as "rebonded urethane foam". This rebonded urethane foam may then be fabricated into whatever shape is required for the particular intended end use application.
Typically, the prepolymers which have been used for such rebonded foam applications comprise the reaction product of an isocyanate such as tolylene diisocyanate with a polyol such as a polyoxypropylene polyol which may contain minor amounts of ethylene oxide on the order of about 15% or less. Prepolymers of this type suitably function to yield the desired rebonded urethane foam product. However, the economy of the manufacture of the rebonded foam product is dependent to a significant extent upon the amount of the prepolymer adhesive which must be used, the time needed to cure the prepolymer adhesive and the time before the rebonded polyurethane foam can be "demolded" (i.e. -- removed from the mold).